1. Technical Field of the Invention
The present invention relates to a control apparatus for an internal combustion engine, which performs ignition control under which high voltage is applied across the electrodes of a spark plug based on electro-magnetic energy stored in a spark coil to produce discharge sparks across the electrodes and, in particular, to a control apparatus for an internal combustion engine, which is able to appropriately determine deterioration as being caused in the spark plug or determine the occurrence of an open failure in an ignition system. Herein, “open failure” means a defective state such that electrical wiring of the circuit is cut thereby the circuit has become opened.
2. Related Art
Due to the recent trend of downsizing vehicles for the purposes of fuel consumption improvement and cost reduction, there is a tendency of using a supercharger to increase a compression ratio in a spark-ignition internal combustion engine (gasoline engine). A high compression ratio raises an in-cylinder pressure (pressure in a cylinder) in a period in which discharge sparks are produced in a gap between a center electrode and a ground electrode of a spark plug. Thus, the spark plug will have high discharge voltage. When the discharge voltage becomes high under the conditions where the electrodes' wear in the spark plug is advanced due to the increase of a running distance or the like, the discharge voltage may exceed an insulation-breakdown threshold voltage of a plug insulator at an early stage, impairing reliability of the spark plug. As a result, discharge sparks would no longer be produced, which may lead to the occurrence of an accidental fire in the internal combustion engine.
In order to cope with this problem, the inventors of the present application paid attention to a technique being disclosed in JP-B-H06-080313. In this technique, a constant-voltage element such as a Zener diode or a Varistor is used in order to restrict the discharge voltage of a spark plug to a predetermined voltage. Specifically, one of two ends of a spark coil is connected to a constant-voltage element that allows a current to pass therethrough when a voltage between a center electrode and a terminal of a spark plug becomes equal to or higher than the predetermined voltage. One of two ends of the constant-voltage element is connected to the center electrode of the spark plug, and anther end is grounded.
According to this configuration, when a voltage applied to the gap of the spark plug is about to exceed the predetermined voltage, the applied voltage is restricted to the predetermined voltage and flattened. Thus, the conditions of the gas in the gap are made suitable for discharge in a period when the applied voltage is maintained at the predetermined voltage, thereby allowing discharge sparks to occur in the gap. With this configuration, the discharge voltage of the spark plug is prevented from becoming excessively high and thus the reliability of the spark plug can be maintained.
As the duration of use of a spark plug becomes longer, the degree of deterioration of the spark plug becomes higher. For example, the gap of the spark plug may be enlarged with the advancement of electrodes' wear of the spark plug. In an ignition system including a constant-voltage element, a higher degree of deterioration of the spark plug means that a longer time is taken accordingly from when the voltage applied to the gap reaches the predetermined voltage until when the conditions of the gas in the gap are made suitable for discharge. Since the electro-magnetic energy stored in the spark coil is finite, the higher deterioration of the spark plug may prevent the conditions of the gas in the gap from becoming suitable for discharge in the period when the voltage applied to the gap is maintained to the predetermined voltage. In this case, discharge sparks are no longer produced, leading to the occurrence of an accidental fire in the internal combustion engine. In order to avoid such a situation, a technique for detecting deterioration of a spark plug is sought for.
For example, an open failure may occur in the constant-voltage element. Specifically, an open failure may occur in an electric path (hereinafter referred to as “constant-voltage path”) extending toward the constant-voltage element from an electric path connecting between the secondary coil and the center electrode. If an open failure occurs, the constant-voltage element loses its function of restricting the voltage applied to the gap. Accordingly, the voltage applied to the gap may exceed an allowable upper limit (upper-limit withstand voltage). This may impair the reliability of the ignition system including the spark plug. In order to avoid such a situation, a technique for detecting an open failure of the constant-voltage path is sought for.